1. Field of the Invention
The present invention relates to a technique for improving switching of a high-power main power supply and, more specifically, to a technique for preventing malfunction of a switching device of low threshold voltage.
2. Description of the Background Art
Switching devices widely used these days are dominantly implemented by Si transistors. Recently, however, a device formed of a wide band-gap semiconductor such as SiC or GaN is attracting attention. The reason for this is that a wide band-gap semiconductor realizes device performance exceeding limits of physical property of Si.
It is not easy, however, to replace an Si transistor with a transistor formed of a wide band-gap semiconductor, as the transistor formed of Si is a normally-off type device while most of the transistors formed of wide band-gap semiconductor are of normally-on type. Here, a transistor in which a current flows between terminals even when control voltage is 0V is referred to as a “normally-on type” transistor, and a transistor in which no current flows between terminals when control voltage is 0V is referred to as a “normally-off type” transistor.
In order to replace a power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) of Si, a normally-off type field effect transistor (hereinafter referred to as an FET) for switching, formed of GaN, for example, has come to be formed. Related technique is disclosed in Japanese Patent Laying-Open No. 2004-242475 (hereinafter referred to as '475 application). It is noted, however, that the threshold voltage of such transistor is about 0.2V, which is not very high.
Assume that a switching device is used for switching a main power supply for an inverter in an air conditioner or an inverter for solar cells. In such a case, the main power supply to be turned on/off by the switching device typically has a voltage of about 500V and supplies a current of about 50 A. When on/off of such high-power main power supply is to be controlled, it is difficult to insert an additional switch formed of a semiconductor or a relay to the current path to be switched. The reason for this is as follows. Even if the switch has a resistance of about 20 mΩ, power consumption by the switch would be 50 W when a current as large as 50 A is caused to flow therethrough, and hence, some measures, such as radiation, must be taken for the heat. Accordingly, it is often the case that a switching device used in an inverter or the like is kept subjected to the voltage from the main power supply.
In this connection, an HEMT (High Electron Mobility Transistor), which is one type of the normally-off type wide band-gap semiconductor FET, is disclosed in T. Kawasaki et al., “Normally-off AlGaN/GaN HEMT with Recessed Gate for High Power Applications”, Ext. Abst. 2005 Int. Conf. on Solid State Devices and Materials, September 2005, pp. 206-207.
At present, however, it is difficult to use HEMT in accordance with Kawasaki et al., as the switching device for a high-power main power supply, because such transistor has low threshold voltage, as mentioned above.
The power supply voltage controlled by the switching device is about tens to hundreds volts (V) and it possibly exceeds 1000V. The current value is a few to tens of amperes (A) and it possibly exceeds 100 A. Turning on/off of such high voltage possibly involves generation of noise voltage equal to or higher than the power supply voltage, at the main power supply. Further, because of capacitive coupling or electromagnetic coupling between the main power supply and the control circuit, high-level noise voltage also generates in the control circuit. If the threshold voltage is low, there is a high risk that such noise voltage causes malfunction of the switching device. In a high-power power supply, other switching circuits and the like are connected in parallel with the main power supply. When these circuits turn on/off the high voltage and high current as described above, noise voltage may be generated in the main power supply. In addition, because of capacitive coupling or electromagnetic coupling between the main power supply and other control circuits in the apparatus, high-level noise voltage also generates in the control circuits.
At present, for a normally-off type switching device of which threshold voltage is 1V or smaller, a practical switching circuit has not yet been studied, and the problem of malfunction of the switching device caused by the noise voltage is not recognized. The inventor studied the influence of noise on a switching circuit including a switching device formed of a normally-off type wide band-gap semiconductor.
FIG. 1 is a circuit diagram of a polarity reversing circuit 10 including a conventional switching device, which shows the principle of malfunction caused by noise in a relatively easy manner. Referring to FIG. 1, polarity reversing circuit 10 includes: a normally-off type field effect transistor 20 formed of GaN, for example, having its drain electrode connected to a main power supply voltage Vdd (hereinafter simply denoted as “Vdd”); and a driving pulse generating circuit 22 for driving FET 20, connected to source and gate electrodes of FET 20. The driving pulse generating circuit includes an output circuit 50 for outputting the driving pulse to FET 20.
FET 20 has a threshold voltage Vth of 0.3V. Driving pulse generating circuit 22 is connected between a power supply voltage Vdd2 (hereinafter denoted as “Vdd2”) for circuit operation and the ground. The main power supply voltage Vdd has the voltage of 500V and the current of 50 A. Vdd2 is a relatively low voltage of about 10 to 20V, which is generated by a separate circuit, from Vdd.
The circuit 10 further includes a resistor 24 for stabilizing gate potential, connected between the gate electrode and the source electrode of FET 20. As the output impedance of driving circuit for FET 20 is a few Ω, a resistor having the resistance value several tens to several hundreds of times higher is used as resistor 24.
The circuit 10 further includes: a diode 28 having its cathode electrode connected to the source electrode of FET 20; an inductor 26 connected between the ground and a node between the source electrode of FET 20 and diode 28; a load 32 connected to the anode electrode of diode 28; and a smoothing capacitor 30 connected between the ground and the anode electrode of diode 28. Here, the load refers to a circuit that operates with the electric power applied from power supply potential Vdd and FET 20.
In FET 20, there is a parasitic capacitance component derived from gate/drain capacitance 42 and gate/source capacitance 44, referred to as a feedback capacitance.
FIG. 2 shows a circuit structure of driving pulse output circuit 50 of driving pulse generating circuit 22 shown in FIG. 1. Referring to FIG. 2, output circuit 50 includes: a normally-off type, P-type MOSFET 56 and a normally-off type, N-type MOSFET 58 having drain terminals connected to each other; an input terminal 52 commonly connected to the gate terminals of these; and an output terminal 54 commonly connected to the drain terminal of these. P-type MOSFET 56 has its source electrode connected to power supply potential Vdd2. N-type MOSFET 58 has its source connected to a source potential.
In polarity reversing circuit 10 shown in FIG. 1, assume that the power supply to driving pulse generating circuit 22 is turned off, while a voltage is constantly applied to Vdd. Referring to FIG. 2, in such a situation, power supply potential Vdd2 comes to have high impedance. Therefore, the source and gate potentials of MOSFETs in driving pulse generating circuit 22 come to be equal to the source potential Vss2 of the switching device. Accordingly, the output of driving pulse generating circuit also comes to be equal to Vss2. As a result, the source, drain and gate potentials of the MOSFETs of driving pulse generating circuit 22 all come to have the same potential. In such a state, the voltage between the source/gate of each of the P-type MOSFET 56 and N-type MOSFET 58 is 0V and, therefore, these normally-off type transistors are turned off, and the output impedance is kept high.
Referring to FIG. 1, the gate electrode and the source electrode of FET 20 are connected by resistor 24. Therefore, the gate/source potential of FET 20 attains to 0V, and PET 20 is turned off. Further, the source potential of FET 20 is 0V, as it is grounded through inductor 26.
Here, assume that a noise signal equal to or higher than the power supply voltage is generated by other switching operation, as described above. If the noise signal has a frequency lower than the switching frequency of the circuit, the impedance of inductor 26 lowers. Inductor 26 does not function as an inductance. The noise voltage derived from the noise signal is divided by a series connection of gate/drain capacitance 42 and parallel connected gate-source capacitance 44, driving pulse generating circuit 22 and resistor 24. The divided noise voltage is applied to the gate electrode of FET 20. As the power supply to driving pulse generating circuit 22 is off, it has relatively high output impedance. Therefore, when considering voltage division, driving pulse generating circuit 22 is substantially negligible.
By way of example, when the main power supply voltage is 500V and the current is 50 A, in FET 20 used in such a circuit, the gate/drain capacitance 42 is about tens of pF and the gate/source capacitance is about hundreds of pF. When driving pulse generating circuit 22 turns FET 20 on, part of the driving current flows to resistor 24 and, therefore, a resistor of small resistance value cannot be used as resistor 24. Therefore, typically, a resistor having the resistance value tens to hundreds of times larger than the output impedance at the time of operation of the driving circuit is used. Accordingly, resistor 24 has a resistance value of hundreds of Ω.
Assume that the gate/drain capacitance 42 is 50 pF, gate/source capacitance 44 is 500 pF, resistor 24 has the resistance value of 200Ω, and the noise signal frequency is 50 kH, which is one-half the general switching frequency of 100 kHz. In this case, because of the influence of gate/source capacitance 44 and resistor 24, a voltage of about 1/320 of noise voltage is applied to the gate electrode. If the noise voltage is the same as the power supply voltage of 500V, it follows that a noise of 1.6V generates at the gate electrode. Though it depends on the noise level, frequency and parasitic capacitance component, the noise voltage would be a few volts (V).
The threshold voltage of FET 20 is 0.3V and therefore, it may possibly be turned on by the noise voltage. Generally, when a power supply voltage of 300V or higher is used, malfunction caused by noise is possible if the threshold voltage is 2V or lower. In this regard, in a conventionally used power MOSFET of silicon having breakdown voltage of about 500V, the threshold voltage is about 2 to about 5V. Therefore, the switching device will not be turned on by such noise.
In polarity reversing circuit 10, if FET 20 should turn on at a frequency lower than the actual operation frequency, inductor 26 would not operate as an inductor. As a result, power supply voltage Vdd would be grounded through inductor 26, and a large current could flow through FET 20. Such malfunction increases the risk of breakdown of FET 20, heat generation of circuit lines and fire caused by overheat of FET 20 resulting from excessive current.
A conventional full bridge inverter circuit 70 also faces similar problems. FIG. 3 is a circuit diagram of a conventional full bridge inverter circuit 70.
Referring to FIG. 3, full bridge inverter circuit 70 includes normally-off type FETs 80, 82, 84 and 86 for switching Vdd, to apply power to a load 90. Normally-off type FETs 80 and 84 have drain electrodes connected to Vdd. FET 80 has its source electrode connected to one terminal of load 90, and FET 84 has its source electrode connected to the other terminal of load 90. FET 82 has its drain electrode connected to the source electrode of FET 80 and its source electrode grounded. FET 86 has its drain electrode connected to the source electrode of FET 84, and its source electrode grounded.
Full bridge inverter circuit 70 further includes a driving pulse generating circuit 88, connected between a power supply voltage Vdd2 for circuit operation and the ground, for applying driving pulses to the gate electrodes of FETs 80, 82, 84 and 86.
Referring to FIG. 3, in full bridge inverter circuit 70, if FETs 80 and 82 are turned on simultaneously because of a noise voltage, Vdd would be short-circuited to the ground through normally-off type FETs 80 and 82. This leads to possible problems such as device breakdown or fire resulting from heat generation in the circuit.
As described above, according to the study made by the inventor of the present invention, when a normally-off type FET having relatively low threshold voltage is used as a switching device in polarity reversing circuit 10 or full-bridge inverter circuit 70, there is a risk of circuit breakdown caused by a malfunction, or an accident resulting from heat generation in the circuit. There is a same concern in a half bridge circuit, chopper circuit and the like.
In order to reliably keep off the switching device, it is necessary to add a negative voltage generating circuit to the circuit and to control the same such that it operates in synchronization with the main power supply, as disclosed in '475 application. In that case, however, the negative voltage is used for turning the gate on/off. Therefore, a negative voltage generating circuit having high current driving capability for driving the gate capacitance at high speed, must be used.
Addition of the negative voltage generating circuit having such high output and the control of the same for coordinated operation with the main power supply lead to much complicated circuit structure and hence to higher cost of the product. Further, it is necessary to constantly supply power to the control circuit in coordination with the main power supply. Therefore, the control circuit continuously consumes power even when it is not in operation. Further, the circuit structure would be significantly different from the conventional power MOSFET, resulting in the necessity of large-scale development of new products.